The present invention relates to a spread spectrum (SS) communication apparatus adapted to operate in accordance with a spread spectrum communication scheme, a surface acoustic wave device for use in a SS demodulator of the communication apparatus, and a surface acoustic wave part for use in the SS demodulator.
In recent years, the spread spectrum communication scheme (SS communication scheme) has been drawing much attention as a personal use communication scheme because of its advantages of being resistant to noise and excellent in secrecy and confidentiality. The SS communication scheme generates a modulated signal by modulating information to be transmitted with a carrier signal, and multiplies the modulated signal by a predetermined code sequence at a predetermined high chip rate to perform the spread spectrum modulation (SS modulation), thereby producing a spread spectrum signal (SS signal) which acts as a transmission signal. For the spread spectrum modulation, a pseudo noise code sequence (PN code sequence), a Barker code sequence, and so on have been used as the code sequence mentioned above, and the SS modulation scheme is classified into a direct sequence (DS) scheme and a frequency hopping (FH) scheme.
In the SS communication scheme as mentioned above, the receiver side requires a demodulator for demodulating a SS signal transmitted thereto. For example, when a SS signal has been SS-modulated in accordance with the DS scheme using a PN code sequence on the transmitter side, the receiver side needs to employ the same PN code sequence as the transmitter side for demodulation. Demodulators usable in this event are classified into a demodulator utilizing ICs and a demodulator utilizing surface acoustic wave devices. Surface acoustic wave devices utilized in such demodulators have been drawing much attention since they can realize a demodulator in a simple configuration at a low cost by using the photolythographic technology.
The surface acoustic wave devices may be classified, by their configuration, into a surface acoustic wave matched filter and a surface acoustic wave convolver. Since the surface acoustic wave convolver allows a PN code sequence to be selected for demodulation, it is particularly suited for applications requiring high secrecy and confidentiality. The surface acoustic wave matched filter, although using a fixed code sequence for demodulation, is advantageous in that its peripheral circuits can be configured correspondingly simpler, and therefore the overall system can be built at a low cost, so that the surface acoustic wave matched filter is drawing attention as a component for demodulator for use in small scale SS communication systems, for example, a private wireless LAN and so on. Thus, surface acoustic wave matched filters have been proposed in a variety of shapes. Such surface acoustic wave matched filters are described for example in JP-A-3-77445, JP-A-5-316074, JP-A-6-21752, and JP-A-7-221670.
Most of demodulators employing surface acoustic wave matched filters as mentioned above support a binary phase shift keying scheme (BPSK scheme) which performs demodulation making use of the fact that the surface acoustic wave matched filter takes two polarities, for example, zero-phase and .PI.-phase. For use in a private wireless LAN, it is desirable that the information transmission rate is as high as possible for providing a larger amount of information. To this end, the modulation scheme is required to support an N-phase modulation scheme, which relies on plural-phase or N-phase modulation, rather than the BPSK scheme. A known demodulator supporting the N-phase modulation scheme employs, for demodulation, N or more surface acoustic wave filters corresponding to possible modulation phase values associated with the N-phase modulation. Such a demodulator is described, for example, in JP-A-7-221670.
FIG. 12 is a schematic diagram illustrating the configuration of a surface acoustic wave matched filter for use in a conventional demodulator. A QPSK scheme can provide a transmission rate twice higher than a BPSK scheme. In FIG. 12, the illustrated surface acoustic wave matched filter comprises a piezo-electric substrate 101 made of quartz, LiNbO.sub.3, or the like; a first input electrode 102 of a first line; a second input electrode 103 of the first line formed at a distance of X1 (corresponding to one (.alpha.1) of four possible phase amounts taken by the QPSK scheme) from the first input electrode 102; an output electrode 104 of the first line; a first input electrode 105 of a second line; a second input electrode 106 of the second line formed at a distance of X2 (corresponding to one (.alpha.2) of the four possible phase amounts taken by the QPSK scheme) from the first input electrode 105; an output electrode 107 of the second line; a first input electrode 108 of a third line; a second input electrode 109 of the third line formed at a distance of X3 (corresponding to one (.alpha.3) of the four possible phase amounts taken by the QPSK scheme) from the first input electrode 108; an output electrode 110 of the third line; a first input electrode 111 of a fourth line; a second input electrode 112 of the fourth line formed at a distance of X4 (corresponding to one (.alpha.4) of the four possible phase amounts taken by the QPSK scheme) from the first input electrode 111; an output electrode 113 of the fourth line; a signal input terminal 114; an output terminal 115 of the first line for extracting a signal at the output electrode 104; an output terminal 116 of the second line for extracting a signal at the output electrode 107; an output terminal 117 of the third line for extracting a signal at the output electrode 110; and an output terminal 118 of the fourth line for extracting a signal at the output electrode 113.
Now, a brief description will be given of a demodulation operation in accordance with the QPSK scheme using the surface acoustic wave matched filter illustrated in FIG. 12. A QPSK modulated signal inputted to the input terminal 114 may take a modulation phase in one of four possible states, i.e., one of phase values .alpha.1, .alpha.2, .alpha.3, or .alpha.4. Then, a correlation peak is provided from the output terminal 115 of the first line in the surface acoustic wave matched filter when the modulation phase is at the phase value .alpha.1; a correlation peak is provided from the output terminal 116 of the second line in the surface acoustic wave matched filter when at the second value .alpha.2; a correlation peak is provided from the output terminal 117 of the third line in the surface acoustic wave matched filter when at the third value .alpha.3; and a correlation peak is provided from the output terminal 118 of the fourth line in the surface acoustic wave matched filter when at the third value .alpha.4. Thus, the QPSK modulated signal can be demodulated by determining which line of surface acoustic wave filter a correlation peak is outputted from.
However, the QPSK scheme needs to discriminate four phase states with phase difference of 90 degrees from one another, and moreover, a conventional demodulator cannot reproduce a QPSK signal unless it employs four surface acoustic wave delay lines having different delay amounts from one another corresponding to respective phase values, thus presenting a problem in that the demodulator is complicated.
The spread spectrum communication apparatus, surface acoustic wave device, and surface acoustic wave part are required to enable the demodulation in accordance with the QPSK scheme in a simple configuration.
It is therefore an object of the present invention to provide a spread spectrum communication apparatus capable of implementing the demodulation in accordance with the QPSK scheme in a simple configuration employing a surface acoustic wave matched filter and surface acoustic wave delay lines, a surface acoustic wave device for the QPSK scheme having a surface acoustic wave matched filter and surface acoustic wave delay lines, and a surface acoustic wave part for use in the QPSK scheme.